I am not sure about bridge mode on ZyXel, but I have a different idea.

If you just need basic router functionality, you can use ZyXel as a modem/router but wireless should be disabled, and use Asus as a wireless access point. Then you can get everything work - basic router and faster wireless connection, (plus more Ethernet ports if you use many wired connections.)

Here is what you might want to consider your choice.
The Asus can perform 450Mbps for 802.11n connections plus 1300Mbps for 802.11ac. However, If you do not have an 802.11ac adaptor on your PC or whatever the device you use, you will never be able to use the 802.11ac one. Even more, if your adaptor supports only up to 300Mbps on 802.11n, you won't get 450Mbps on 802.11n either.

I don't have any picture about your usage and environment, but I just feel that you might not need such an advanced wireless connection for now unless you transfer big files between PCs (or between PC and NAS etc.) within LAN frequently. In the case, you can benefit from the faster wireless connections (only if your devices support 802.11ac as I mentioned above.)